1. Field of the Invention
The present invention relates to measurement apparatuses that obtain displacement information by converting relative displacements of objects into periodic signals and counting the periodic signals, and also relates to multi-axial displacement measurement apparatuses that measure displacements in multiple directions.
2. Description of the Related Art
Known examples of a displacement measurement apparatus that measures relative displacement of a mechanical stage employ optical encoders and laser interferometers.
An exemplary displacement measurement apparatus that measures displacement along a single axis is disclosed in Japanese Patent Laid-Open No. 6-18291 (see FIG. 4 therein). This apparatus is an interference encoder, and causes a 0th-order diffracted light beam and a +1st-order diffracted light beam resulting from light transmitted through a diffraction grating to be incident on another diffraction grating provided on a scale attached to an object to be measured, whereby a +1st-order reflected diffracted light beam resulting from the 0th-order diffracted light beam and a −1st-order reflected diffracted light beam resulting from the +1st-order diffracted light beam interfere with each other. The intensity of interference light varies with the amount of displacement. Therefore, displacement of the object is measured by detecting the intensity of interference light with a photodetector.
Another displacement measurement apparatus is disclosed in Japanese Patent Laid-Open No. 2007-171206 (see FIGS. 1 and 19 therein). This apparatus measures displacement in one direction (along the X axis) within a plane in which a diffraction grating that moves together with an object to be measured is provided, and displacement in another direction (along the Z axis) orthogonal to the plane. The displacements in the X and Z directions, are measured with respective measurement systems, each including a light source, an interferometer core, a polarization conversion device, a detector, and so forth. Specifically, two measurement systems are provided in parallel, and two light beams are applied, at Littrow angles, to a diffraction grating, whereby reflected diffracted light beams are produced. Subsequently, two pieces of displacement information as composite information with respect to the X and Z directions, i.e., Z+X information and Z−X information, are obtained. Thus, X-direction displacement information and Z-direction displacement information are calculated.
Japanese Patent Laid-Open No. 2007-171206 also discloses a triaxial displacement measurement apparatus that measures displacements in two directions (along the X and Y axes) within a plane in which a diffraction grating that moves together with an object to be measured is provided, and displacement in another direction (along the Z axis) orthogonal to the plane. Also in this case, the same measurement principle as in the biaxial measurement along the X and Z axes applies. That is, three measurement systems are used so as to obtain three pieces of displacement information, i.e., Z+X information, Z−X information, and Z+Y information. Thus, X-direction displacement information, Y-direction displacement information, and Z-direction displacement information are calculated.
Another displacement measurement apparatus that measures displacements in two directions within a plane in which a diffraction grating that moves together with an object to be measured is provided is disclosed in Japanese Patent Laid-Open No. 6-288722 (see FIG. 1 therein). This apparatus measures displacements of the object in two directions (along the X and Y axes) that are orthogonal to each other within the plane. A diffraction grating having a checkerboard pattern and configured to move together with the object is provided in a plane containing the X and Y axes. Light is made to be orthogonally incident on the diffraction grating. Light transmitted through the diffraction grating and diffracted in the X direction produces a +1st-order diffracted light beam and a −1st-order diffracted light beam, which are further made to be incident on respective incidence diffraction gratings. The +1st-order diffracted light beam and the −1st-order diffracted light beam are introduced from the incidence diffraction gratings into respective strip waveguides provided in the XY plane, and are subsequently made to interfere with each other at a coupler. By detecting the resulting interference light with a detector, X-direction displacement is measured. The same applies in Y-direction measurement: light is diffracted in the Y direction into a +1st-order diffracted light beam and a −1st-order diffracted light beam, which are introduced into respective strip waveguides and are subsequently made to interfere with each other at a coupler.
The displacement measurement apparatus disclosed in Japanese Patent Laid-Open No. 2007-171206 requires a plurality of measurement systems, each including a light source, a polarization beam splitter, a deflector, a photodetector, and so forth, so as to measure displacement in a direction defined within the plane where the diffraction grating is provided and displacement in a direction orthogonal to the plane. Therefore, the apparatus becomes larger and more complicated. Moreover, in the case where a plurality of measurement systems are provided, since positions where measurements are performed vary with different axes, errors may occur particularly when high measurement accuracy is required. In addition, mathematical operations performed by a field-programmable gate array (FPGA), a central processing unit (CPU), or the like include separation operation, preventing quick signal output. For these reasons, such an apparatus may be impractical for use in stage control.
The displacement measurement apparatus disclosed in Japanese Patent Laid-Open No. 6-288722 measures displacements in two directions within the plane where the diffraction grating is provided, by introducing the plurality of diffracted light beams, produced by light transmitted through the diffraction grating, into the strip waveguides provided in a plane parallel to the diffraction grating. However, to cause the plurality of diffracted light beams that are guided respectively to interfere with each other on an actual mechanical stage, problems such as measurement errors due to modulation of the light beams during guiding may occur. In addition, the diffraction grating configured to move together with the object is a transmissive component, and therefore is provided between the light source and the incidence diffraction gratings. This increases the size and complexity of the apparatus.